1. Field of the Invention:
Polymer blends containing a polycarbonate (PC) and one or more block copolymers having the general structures: A-b-B (diblock); A-b-B-b-A (triblock); B-b-A-b-B (triblock); or (A-B).sub.n (multiblock) as examples, where the A block is a polymer of an aromatic(alkyl)methacrylate (PAAM) such as polyphenyl methacrylate, polybenzyl methacrylate or polyphenylethyl methacrylate; the B block is a rubbery polymer such as polyisoprene (PiP), polybutadiene (PBD), polylauryl methacrylate (PLM), siloxane rubber or polybutyl acrylate (PBA); and "b" indicates a block structure. When thoroughly mixed with polycarbonate the block copolymers enhance thick section toughness and low temperature impact strength without impairing transparency.
2. Discussion of the Background:
The present invention pertains to PC/PAAM-b-B binary blends, as well as ternary blends containing up to 15% of additional PAAM and/or B homopolymers. These compositions contain a thermodynamically miscible, single phase of PC and PAAM, plus very finely dispersed particles of the B block in a separate phase, which are usually on the order of 40 to 2,000 angstroms and thus much smaller than the wavelength of light. The combined effects of the single phase nature of the PC/PAAM mixture plus the very small size of the dispersed second block leads to an optically transparent material. Furthermore, the chemical attachment of the B block to the PAAM block assures perfect adhesion and translation of physical properties from the B block to the PC/PAAM phase. Thus, one can obtain a mixture which combines the advantageous properties of two dissimilar materials and still maintain transparency. A fine and stable dispersion that retains transparency can only be obtained through the use of block copolymers which have one block that is thermodynamically miscible with the polycarbonate. Thus, the basis of this invention is two-fold: 1) the synthesis of block copolymers having one block that is thermodynamically miscible with PC and another block capable of improving some deficiency of polycarbonate; and 2) preparation of blends of such block copolymers with PC (and optionally, additional corresponding homopolymers) which exhibit improved properties over polycarbonate alone, yet retain the benefit of its clarity.
Aromatic polycarbonates exhibit good thermal stability, good dimensional stability, good impact strength in thin sections, relatively good stiffness and most notably good transparency. For these reasons, PC is used in a variety of applications including glass replacement, housings, medical devices and containers. Nevertheless, PC does have drawbacks such as poor scratch resistance, poor long term U.V. stability and poor stress birefringence which must be dealt with, especially in demanding optical applications. Moreover, it is often desirable to improve the processability, thick section toughness and low temperature impact strength of PC without sacrificing its transparency.
Block copolymers are a general class of materials that exhibit a wide range of properties and are unique in their ability to "microphase separate" which refers to a fine separation of the two dissimilar polymer blocks into distinct phases.
Methacrylic ester based polymers suffer from poor dimensional stability and poor heat distortion yet have good clarity, surface hardness, U.V. resistance and processability. For this reason they are commonly used in applications such as window glazings, aircraft windows and automotive lenses and lightcovers. Thus, blends of PC and methacrylic polymers should have a good balance of properties and, if they formed a single phase, would also be clear. Unfortunately such blends, even if transparent, would still suffer from poor low temperature impact strength and poor thick section toughness. The simple addition of a rubber impact modifier would greatly improve such deficiencies but at the certain impairment of transparency because rubbery impact modifiers are notoriously incompatible with both PC and methacrylates. Thus the task existed of finding a methacrylic ester-based polymer which is thermodynamically miscible with polycarbonate and which can be block copolymerized with an impact modifier, such as polyisoprene, to prepare a blend of the two which exhibits not only the improvements gained from the blending of PC and the methacrylate polymer but also has improved thick section toughness and low temperature impact strength, all while retaining the inherent transparency of polycarbonate.
The term "thermodynamically miscible" is known in the art to define a polymer blend that mixes on the molecular level so as to form a single, homogeneous phase which exhibits only one glass transition (Tg). The term is used in comparison to the term "mechanically compatible" which is taken to mean that mixing of the polymers is on a small scale but larger than the molecular level. Furthermore, mechanical compatibility implies that the multiple phases exhibit good adhesion to one another providing good mechanical properties. Although both thermodynamically miscible and mechanically compatible blends exhibit good mechanical properties, only thermodynamically miscible blends are transparent owing to their single phase nature.
Although blends of PC with methacrylic polymers are often compatible resulting in improvements over their respective components, most are not miscible and their opacity makes them unacceptable in optical applications. For example, U.S. Pat. No. 4,319,003 teaches that blends of PC and polymethyl methacrylate (PMMA) are opaque and often do not possess, the advantageous properties expected of such mixtures. Among other references that report the immiscibility of PMMA with PC are JP 7216063 and EP 0297285.
Ways to overcome the immiscibility of typical PC/PMMA mixtures have, however, been disclosed. The most commonly employed method is the addition of comonomers to the PMMA (DE 2264268; DE 3632946; and U.S. Pat. No. 4,906,696). Recently a number of patents and publications have appeared which demonstrate the miscibility of PC with random copolymers containing methylmethacrylate and either cyclohexyl methacrylate or phenyl methacrylate. The thermodynamic miscibility of PC with pure polyphenyl methacrylate was also reported. EP 0297285; U.S. Pat. No. 4,906,696; J. Appl. Polym. Sci. 44, 2233-2237, 1991; and Polymer 32(7) , 1274-1283 (1991). This miscibility property Was used to increase the adhesion between a rubbery impact modifier, ethylene-propylene-diene (EPDM), and polycarbonate. In that case, phenyl methacrylate was grafted onto EPDM. However, the resulting graft copolymer was mechanically compatible with PC but not thermodynamically miscible, as might have been hoped.
U.S. Pat. No. 4,997,883 and EP 0326938 both teach the art of grafting aromatic(meth)acrylate/methylmethacrylaterandom copolymers onto a preexisting EPDM polymer to prepare an elastomeric graft copolymer which, when added to PC, shows improvement in impact strength. Unfortunately, all of these materials are also opaque. Thus, the task still existed to develop a means of blending PC with impact modifiers without simultaneous loss of clarity. I have discovered that this goal can be achieved with a block copolymer containing a polyaromatic(alkyl)methacrylate block and a second block consisting of the impact modifier. When thoroughly blended with PC, the mixtures produce transparent, modified PC-based materials, in contrast to the opaque mixtures of the prior art based on EPDM graft copolymers. Thus, the present invention represents a distinct improvement over the technique where PC-miscible polymers are grafted onto an impact modifier.
In addition, I have found that polybenzyl methacrylate and polyphenylethyl methacrylate are completely miscible in all proportions with polycarbonate, and that copolymers containing blocks of these polymers have the same property.
A general synthesis of well defined methacrylic ester-containing block copolymers has only recently been accomplished (See for example: "Recent Advances in Mechanistic and Synthetic Aspects of Polymerization", Kluwer Academic Publishers, Norwell, Mass., 1987; and "Recent Advances in Anionic Polymerizations", Elsevier Publishing Co., New York, N.Y., 1987). These reports have focused primarily on polymers containing blocks of polymethyl methacrylate or polybutyl methacrylate made by an anionic mechanism. Typically, anionic polymerization is used for the synthesis of well defined block copolymers because the reaction has no naturally occurring termination step. However, the presence of carbonyl groups initially caused problems with the polymerization of methacrylate monomers until methods were developed to prevent attack on the carbonyl groups. The most commonly accepted method is the combined use of low temperature polymerization (-78.degree. C.) and modification of the initial anion, either by prereaction with 1,1-diphenylethylene or by variation of its reactivity by reaction/chelation with pyridine and/or LiCl. Within the above cited references, no method for the anionic polymerization of polyaromatic(alkyl)methacrylates (PAAM) is given nor a synthesis of well defined block copolymers containing PAAM blocks. Thus the task still existed of developing a method for synthesizing methyl methacrylate/PAAM block copolymers.